This invention relates to a system for producing heat at elevated temperatures by upgrading heat available from a lower temperature sensible heat source, and more particularly to the use of a combined Rankine cycle evaporator and absorption heat pump system having one or more stages which efficiently sensible waste heat from industrial or other sources to boost a portion of that heat to a useful level.
Many industries must dispose of large amounts of heat either produced as a by-product of a chemical reaction or through the operation of heat exchange process equipment because that heat is at too low a temperature for further use. The heat may be available as latent heat (i.e., low pressure steam) or sensible heat (i.e., hot water). Sources of this wasted heat include heat losses from boilers, drying equipment, chemical reactors, and the like and heat derived from heat exchange equipment. In many cases, substantial amounts of increasingly expensive fuel must be burned only to result in much of the heat produced being lost as waste heat. If a portion of this waste heat could be upgraded for further use, energy would be conserved and fuel cost savings realized.
A heat pump can be used to increase the temperature of waste heat (available as low pressure steam or heated fluid) to a useable level. For example, a vapor compression heat pump cycle can be operated using a compressor which provides high pressure fluid to a condenser. As the fluid condenses, it gives up heat (e.g., heat of condensation) at a temperature level higher than that of the fluid supplied to the compressor. After being throttled to an evaporator, the fluid vaporizes by accepting heat from an external source (e.g., ambient heat sink) and is recycled to the compressor.
However, work is required to operate the compressor, and in most cases this work is obtained from the combustion heat of fuel. Where the temperature boosts are modest, a savings in energy can be achieved when an efficient advanced power conversion system is employed. Where the power is supplied to the compressor from a central source such as electric power to drive an electric motor, the original source of energy can be a dirty fuel such as coal requiring costly pollution control, or an advanced fuel such as nuclear requiring careful safety monitoring. Attempting to reduce overall costs by burning clean fuels on site is offset by the system complexity and represents only a marginal savings at best.
It has been suggested to utilize some of the waste heat available in a heat engine to drive the compressor in a vapor compression heat pump cycle. For example, U.S. Pat. No. 4,089,186 teaches such a process in which at least a portion of a working fluid is vaporized utilizing the heat available from a waste heat source. A turbine runs a compressor which condenses the vapor. The heat of condensation of the vapor is then utilized to raise the temperature of a second fluid in a heat exchange zone. However, such a process still requires the use of a compressor and a turbine to run the compressor. Not only are these pieces of equipment expensive to buy and operate, but they also have a high loss of energy due to mechanical inefficiencies.
In order to eliminate the need for a compressor and turbine, an absorption cycle heat pump process may be utilized. In an absorption cycle, a secondary fluid, termed the absorbent, is employed to absorb a primary fluid, termed the refrigerant, which has been vaporized in an evaporator. In a conventional simple absorption heat pump cycle, heat from an external source is utilized in a desorber to produce a relatively high pressure refrigerant vapor which is then taken to a condenser. Heat which can be utilized is released upon condensation of the vapor. The liquid refrigerant is then vaporized using heat from a relatively low temperature external source and reabsorbed into the absorbent and an absorber where heat which can also be utilized is released. The refrigerant-absorbent solution is then pumped to the desorber to repeat the process. The use of a desorber and absorber eliminates the need for a compressor in the cycle but still requires high quality heat for operation.
A modification of a conventional absorption cycle heat pump is taught in U.S. Pat. No. 4,167,101 as a means to elevate the temperature of a waste heat source. In that substantially isobaric process, a vapor is absorbed into a liquid phase solvent in an absorption zone and releases its heat of solution to an external heat receiving medium. The solution is then taken to a stripping zone where a stripping gas desorbs the vapor from solution.
The resulting gaseous mixture is then fractionated by partial liquefaction and phase separation. The stripping gas is then recycled to the stripping zone while the liquid fraction is vaporized and then recycled to the absorber where the process is repeated. Although this process eliminates the need for a mechanical compressor, it has several disadvantages. The system is made more complex by the addition of a third component, a stripping gas, to it. Moreover, rather than having simple condensation and evaporation stages, the process requires partial liquefaction and phase separation to separate the stripping gas from a portion of the working fluid.
Williams and Tiedemann, in "A Heat Pump Powered by Natural Thermal Gradients," a paper presented at the 9th IECE Conference, Aug. 26-30, 1974, have suggested using a solution heat pump cycle and the temperature gradients that exist in subarctic climates between sea water and the ambient air to generate heat for residential housing. That proposal utilized an ammonia-water working fluid, a relatively high pressure absorber, and a relatively low pressure desorber as the solution heat pump. However, because of the low efficiency of the process and the requirement of a large and costly physical plant, the authors concluded that the system was impractical for its designed use.
Accordingly, the need exists in the art for an efficient heat boosting system which can upgrade waste heat from industrial and other sources.